如何提高这个 QUERY3 的运行效率？

Table articles:
id|category_id|title|created|updated
Index:
ADD KEY `category_id` (`category_id`,`created`)

My testing:

query1

SELECT sql_no_cache * FROM `articles` WHERE category_id=1 order by created limit 0,15
Average time: 0.0003

query2

SELECT sql_no_cache * FROM `articles` WHERE category_id=1 order by updated limit 0,15
Average time: 0.0019

query3

SELECT sql_no_cache * FROM `articles` WHERE category_id in (1,2,3) order by created limit 0,15
Average time: 0.0018

query4

SELECT sql_no_cache * FROM `articles` WHERE category_id in (1,2,3) order by updated limit 0,15
Average time: 0.0018

created 做了二级索引， query1 明显用了索引（对比 query2 ），但是 query3 没用索引。请问如何索引提高 query3 的查询效率？